Every object emanates or reflects its own light and has a different color depending on a wavelength of the emanated or reflected light. Each color of visible light has a different wavelength. For example, red light has a longest wavelength in the visible light, while violet light has a shortest one. Further, the visible light penetrates objects to different depths depending on the wavelength of the visible light. In particular, the object penetration depth of the visible light is in proportion to the wavelength of the visible light.
The above-mentioned characteristics are applied to an image sensor device. A conventional image sensor using a charge coupled device (CCD) has drawbacks such as a complicated fabrication process, a low yield and a high manufacturing cost. In order to solve such problems, a CMOS image sensor manufactured by using a CMOS technology is suggested.
Generally, known CMOS image sensors include a plurality of pixel units having a light sensing region and an active region. Each of the pixel units also includes a light sensing element formed on the light sensing region and a plurality of transistors formed on the active region. The light sensing elements, such as photodiodes, sense incident light reflected from the object and accumulate photoelectric charges that are generated by the incident light. The transistors control the transfer of the photoelectric charges.
One example of a known CMOS image sensor is described below with reference to FIGS. 1 and 2. FIG. 1 shows a plan view illustrating the pixel units of the known CMOS image sensor. In FIG. 1, a pixel unit is referred to as pd-A and pixel units on the upper and lower side of the pixel unit pd-A are referred to as pd-AU and pd-AB, respectively.
As shown in FIG. 1, each pixel unit (pd-A, pd-AU and pd-AB) includes the light sensing element 1 (e.g., photodiode), for converting incident light into electrical signals and the active region 2 for managing or externally transmitting the electrical signals generated in the light sensing element 1. In this configuration, the pixel units (pd-A, pd-AU and pd-AB) are located next to one another.
In an arrangement, such as the arrangement of FIGS. 1 and 2, when the pixel unit pd-A is exposed to incident light, electrical signals generated in the light sensing element 1 of the pd-A affect the pixel units (pd-AU and pd-AB) adjacent thereto. Such a phenomenon is referred to as crosstalk.
The crosstalk also affects the active regions 2 of adjacent pixel units (pd-AU, pd-AB) of the pd-A through right and left adjacent active regions of the light sensing element 1 of pd-A. In this case, the crosstalk can be avoided by controlling voltage applied to the active region 2.
However, because each sensing element 1 of the pd-AU and the pd-AB is adjacent to the sensing element 1 of the pd-A, electrical signals generated of the pd-A affect the pixel units (pd-AU and pd-AB). That is, crosstalk occurs between the adjacent pixel units, and thereby deteriorate characteristics of the pixel units pd-AU and pd-AB.
The problem of the crosstalk is described with reference to FIG. 2, which depicts a cross-sectional view taken along A-A′ shown in FIG. 1. In the conventional CMOS image sensor, impurities implanted in the pd-A are diffused through bottom portion of element isolation layer 4, or a depletion layer between pixel units is extended due to a potential difference, electrical signals generated in the pd-A can easily move to neighboring pixel units through the bottom portions of the element isolation layers 4. As a result, electrical signal generated in the pd-A affects the pd-AU and the pd-AB, and thus crosstalk is occurred between adjacent pixel units.
One known solution to solve the above shortcoming is to inject impurities into a portion that is located within the light sensing region and having a predetermined distance from the element isolation layers 4 to form a photodiode therein. However, such a solution also has limits due to a decrease in photodiode region within the light sensing region and a gradual high integration of a semiconductor device.